the tables for fcr on trangs form are;
the FPA is the number of frames per animation, limiting frequency of casts
the AF is the action frame, how soon into the animation the action takes place
vampire form has the worst FPA in the game but the best AF in the game- even better than a 200% fcr sorc- meaning you can always get off one cast very very quickly, but then you're stuck "self-stunned" as you wave your arms uselessly in the air after casting in the backswing of the animation
this however works quite well if you can interrupt your backswing animation somehow. Its been possible with weapon switching and some tricks, but blizzard has changed the behavior on the serverside several times and I don't know what works and what doesn't anymore since there's no documentation. But at least it
was possible to take a trangs necro into pvp and effectively take only ~4-8 frames to cast a spell and immediately resume running via WSG, canceling your backswing entirely. Perhaps with hax it was even possible for a trangs nec to cast at 4 fpa, like modders once did using tmc- but again, patched on serverside, so who knows what happens now